1. Field of the Invention
The present invention relates to a duplex speech transmission method and a system therefor. The present invention has particular, but not exclusive, application to demand-assigned time division duplex speech transmission systems which may be used in cordless telephone systems.
2. Description of the Related Art
Time division speech duplex systems are known; see, for example, the article by Henry, P. S. and Glance, B. S. "A new approach to high capacity digital mobile radio". Bell System Technical Journal, October 1981, pp 1891 to 1904. The mode of operation of one such system is illustrated in FIG. 1 of the accompanying drawings. For the sake of illustration it will be assumed that there is a base station having a transceiver capable of duplex operation and a handset having a similar type of transceiver. Speech emanating from the handset and the base station is split into time segments 12, the base station speech signal being shown in the diagram referenced 1(i). As the time period on the communication channel has to be used for transmission from both transceivers or either one of them, then in order to do this the segments 12 of speech are time-compressed, usually digitally, by a factor of 2 and transmitted as a time division duplex signal. This is illustrated in diagram 1(ii) wherein alternate slots 14 relate to transmission from the handset (H) to the base station (B) and the intervening slots 16 to transmissions from the base station to the handset. The cross-hatched portions 18 between the slots 14, 16 are guard-bands to allow time for the radio equipment to switch from transmit to receive and vice versa. At the handset, diagram 1(iii), the time-compressed base station speech is re-expanded to fill a time segment 19 of the same length as the segment 12. Although not shown, a similar expansion of the handset speech takes place at the base station.
In time division duplex systems applied to telephone systems it has been found that during a typical call each party speaks for less than 50% of the time. Accordingly, to allocate time slots in this way is clearly a waste of available capacity in a transmission channel if there is no information to be transmitted.
A duplex voice communication system is known from British Patent Specification No. 1440047 in which segments of speech are first stored alternately in analogue stores and subsequently the stores are read-out alternately and the segment of speech being read-out is then quantised at a high or low rate depending on whether one or both parties originating the speech want to transmit simultaneously. The quantised speech is then transmitted on a limited bandwidth channel in one of a plurality of successive time slots of 50 milliseconds (mS) duration. In all two time slots (100 mS) are required to complete the transmission of a segment of speech. Such a system is not suitable in cordless telephone systems for use on the Public Switched Telephone Network (PSTN), which only permits delays of the order of 1 mS, requiring 2 mS time slots. It is estimated that to adapt the known system for use on the PSTN would be impractical because the signalling overheads relative to the digitised speech would be too great. Furthermore the use of analogue and digital circuitry mitigates against fabrication as an integrated circuit.